Service Tax on Flats if Builder does not have Registration

I booked a flat in February, 2015 in Kolkata. I have made 80% payment till March, 2016. In the month of April, 2016 I have done the rest 20% payment and took the possession. The registration is yet not done. The Registration is going to be done this month (August, 2016).

The builder is not having any service tax registration. Now the builder has applied for the service tax registration. The builder is asking us to pay the service tax @ 3.75 on the total amount we paid.

I want to know that whether he has the power and authority to collect the tax on the amounts which were already paid and the money receipts were also issued at the time of payment. The payment belong to F.Y. 2014-15, 2015-16 and 2016-17.